MCP9800A0-MSLASHMS datasheet(27/42 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

DS21909C-page 27

MCP9800/1/2/3

6.0

APPLICATIONS INFORMATION

6.1

Connecting to the Serial Bus

The SDA and SCL serial interface are open-drain pins

that require pull-up resistors. This configuration is

shown in Figure 6-1.

FIGURE 6-1:

Pull-up Resistors On Serial

Interface.

The MCP9800/1/2/3 is designed to meet 0.4V

(maximum) voltage drop at 3 mA of current. This allows

the MCP9800/1/2/3 to drive lower values of pull-up

resistors and higher bus capacitance. In this

application, all devices on the bus must meet the same

pull-down current requirements.

6.2

Typical Application

Microchip provides several microcontroller product

lines with Master Synchronous Serial Port modules

(MSSP) that include the I2C interface mode. This

module implements all master and slave functions and

simplifies

the

firmware

development

overhead.

Figure 6-2 shows a typical application using the

PIC16F737 as a master to control other Microchip

slave products, such as EEPROM, fan speed

controllers and the MCP9800 temperature sensor

connected to the bus.

FIGURE 6-2:

Multiple Devices on I2C™

Bus.

The ALERT output can be wired with a number of other

open-drain devices. In such applications, the output

needs to be programmed as an active-low output. Most

systems will require pull-up resistors for this

configuration.

6.3

Layout Considerations

The MCP9800/1/2/3 does not require any additional

components besides the master controller in order to

measure temperature. However, it is recommended

that a decoupling capacitor of 0.1 µF to 1 µF be used

between the VDD and GND pins. A high-frequency

ceramic capacitor is recommended. It is necessary for

the capacitor to be located as close as possible to the

power pins in order to provide effective noise

protection.

For applications where a switching regulator is used to

power the sensor, it is recommended to add a 200Ω

resistor in series to VDD to filter out the switcher noise

from the sensor. It is also recommended to add the

series resistor in applications where a linear regulator

is used to step-down a switching regulator voltage to

power the sensor. For example, if a linearly regulated

3.3V from a 5V switching regulator is used to power the

sensor, add a 200Ω series resistor (refer to Figure 6-3).

FIGURE 6-3:

Power-supply Filter using a

Single Resistor.

6.4

Thermal Considerations

The

MCP9800/1/2/3

measures

temperature

monitoring the voltage of a diode located in the die. A

low-impedance thermal path between the die and the

Printed Circuit Board (PCB) is provided by the pins.

Therefore, the MCP9800/1/2/3 effectively monitors the

temperature of the PCB. However, the thermal path for

the ambient air is not as efficient because the plastic

device package functions as a thermal insulator.

A potential for self-heating errors can exist if the

MCP9800/1/2/3 SDA and SCL communication lines

are heavily loaded with pull-ups. Typically, the

self-heating error is negligible because of the relatively

small current consumption of the MCP9800/1/2/3.

However, in order to maximize the temperature

accuracy, the SDA and SCL pins need to be lightly

loaded.

PIC®

SDA

SCL

VDD

MCU

MCP9800/1/2/3

SDA SCL

PIC16F737

Microcontroller

Temperature

Sensor

24LC01

EEPROM

TC654

Fan Speed

Controller

TCN75A

VDD

200



MCP9800/1/2/3

Switching

Regulator

0.1 µF

bypass

VDD

200



MCP9800/1/2/3

Switching

Regulator

0.1 µF

bypass

Regulator

Linear

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